1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and more particularly to a control system for an internal combustion engine for realizing quick activation of a catalyst converter when the internal combustion engine is cold.
2. Description of the Related Art
It is recently required for engine emission control to maintain a catalyst converter in an exhaust pipe of an internal combustion engine always at an adequate active state. For instance, it is desired to shorten engine warm-up time and to activate the catalyst converter quickly in warming up the catalyst converter from the cold state in starting an engine at low temperature.
In JP-A-61-232317, it is proposed to advance timing of opening an exhaust valve of an internal combustion engine to supply exhaust gas having a large amount of heat energy to an exhaust pipe to raise exhaust gas temperature. In JP-A-61-190118, it is proposed to advance valve timing of an exhaust valve when an internal combustion engine is cold to reduce combustion gas expansion ratio and to raise exhaust gas temperature.
However, those conventional systems are directed to raise the temperature of the exhaust gas just by thermal energy of combustion gas within a cylinder. The effect of raising the temperature does not necessarily result in activating the catalyst quickly. Although a start catalyst whose capacity is relatively small has been put into practical use in order to activate the catalyst quickly, such a quick activating type catalyst has been also required to be able to activate more quickly.